A conventional screw 1 as shown in FIG. 1 includes a shank 11 with a tapered end, a thread 12 formed around the shank 11, and one chip collection channel 13 extending from the tapered end of the shank 11 for collecting wood chips when the screw penetrates into a wood and produces wood chips. In FIG. 2, the conventional screw 1 has two chip collection channels 13.
Another conventional screw 2 as shown in FIG. 3 includes a shank 21 having a tapered end in a quadrangular pyramid shape, and a thread 22 formed around the shank 21. A plurality of wavy channels 221 are formed at intervals in the thread 22 and extend from the tapered end of the shank 21, so that the tapered end of the shank 21 is advantageous for cutting and penetrating.
However, when the conventional screw 1 or 2 is forced to penetrate into a wood without a preformed hole, because friction between the screw and wood gradually increases, application force has to be increased to enable the screw to smoothly advance into the wood. In addition, because the chip collection channel 13 or the wavy channels 221 is provided only in a section of the screw from the tapered end, when the amount of wood chips in the chip collection channel 13 or the waved channels 221 increases the wood chips may squeeze into the wood causing the wood to split.
Referring to FIG. 4, a self-tapping screw 3 disclosed in Taiwanese Utility Model Patent No. M460950 includes a shank 31, a thread 32 formed around the shank 31, and a chip collection groove 33 indented into the shank 31 and extending from a tapered end to a head of the self-tapping screw 3. The thread 32 includes a plurality of spaced-apart tooth portions 321 disposed around the shank 31. The chip collection groove 33 includes a plurality of indentation portions 331 each of which is disposed between two adjacent ones of the tooth portions 321. While the self-tapping screw 3 may accommodate an increased amount of wood chips through the indentation portions 331 of the chip collection groove 33, the increased wood chips are unable to be discharged outwardly from the chip collection groove 33.
Referring to FIG. 5, a self-tapping screw 4 disclosed in Taiwanese Invention Patent No. 201013060 includes a shank 41, a thread 42, and a cutting groove 43 helically indented into the shank 41 and the thread 42. When the self-tapping screw 4 advances into a wood, the cutting groove 43 can discharge the wood chips outwardly from the wood. However, the cutting groove 43 is arcuated in cross section. During forming the thread 42 and the cutting groove 43, because the arcuated cutting groove 43 may be unable to mesh with a thread forming die, a positional deviation may occur between the cutting groove 43 and the thread forming die lowering the quality of forming the thread 42.